Most heating systems depend on convection of hot exhaust gases to draw combustion air into the furnace and to discharge the exhaust gases to the atmosphere. In recent years, primarily since the concern over energy shortages and the increased cost of energy, some domestic heating systems have been equipped with a blower to induce the circulation of air through the combustion chamber and to exhaust the products of combustion to the atmosphere. This system gains efficiency because the optimum quantity of combustion air, technically termed the stoichiometric air/fuel ratio, can be more readily controlled and also because it avoids the heat loss that occurs with a natural convection system which tends to draw warm air out of the furnace room even when the system is not operating.
More advanced versions of the force draft system reclaim some of the heat in the exhaust gases through an auxiliary heat exchanger of some type, as shown in U.S. Pat. Nos. 3,813,039, 3,934,798 and 4,241,874. The exhaust gases generally leave the combustion chamber at temperatures in the range of approximately 450.degree. F. This is reduced in the auxiliary heat exchanger and the heat derived from this heat exchanger is then used in one manner or another.